1. Field of the Invention
The present invention relates to a method of correcting an output image signal in a color image reading apparatus which has three R, G, B linear image sensors, each comprising an array of photoelectric transducer pixels arranged in a main scanning direction, for photoelectrically reading reflected or transmitted light which represents image information carried by a subject.
2. Description of the Related Art
Color image reading systems operate by applying illuminating light to a subject placed on a subject table, guiding light representing image information carried by the subject as reflected or transmitted light to a focusing optical system, supplying the light to tricolor separating optical systems such as tricolor separating prisms or the like, and photoelectrically reading the light with R (red), G (green), B (blue) linear image sensors mounted on respective exist surfaces of the tricolor separating prisms.
The subject is read in a main scanning direction by the linear image sensors while at the same time the subject is moved relatively to the linear image sensors in an auxiliary scanning direction that is substantially perpendicular to the main scanning direction, so that two-dimensional image information of the colors R, G, B can be produced.
As schematically shown in FIG. 14 of the accompanying drawings, a general linear image sensor 1 basically comprises a photodetector 2 comprising a linear array of photoelectric transducer elements (hereinafter referred to as "pixels") P arranged in a longitudinal (main scanning) direction and a pixel transfer unit 3 comprising an odd-numbered pixel transfer array 3o and an even-numbered pixel transfer array 3e which are positioned one on each side of the photodetector 2. The pixels P include odd-numbered pixels Po and even-numbered pixels Pe.
Light L applied to the linear image sensor 1 is detected by the photodetector 2 and converted into electric charges by the respective pixels P. The electric charges are successively shifted from the odd- and even-numbered pixels Po, Pe to the corresponding odd- and even-numbered pixel transfer arrays 3o, 3e in response to shift pulses that are periodically generated at constant time intervals. Thereafter, the electric charges are outputted as odd- and even-numbered pixel signals So, Se from odd- and even-numbered output sections 4o, 4e of FDAs (floating diffusion amplifiers) or the like through respective output terminals of the linear image sensor 1.
The odd- and even-numbered pixel signals So, Se are alternately read through a CDS (correlated double signal sampling) circuit and an amplifier (not shown), and converted by an A/D (analog-to-digital) converter (not shown) into digital image signals, which are then alternately introduced by a selector switch and stored in a line memory or the like as one-line image data corresponding to the photodetector 2.
If the resolution of the color image reading system with the tricolor separating optical system is to be increased, then the color image reading system may basically use a linear image sensor having many photoelectric transducer pixels P. Recently, linear image sensors having more than 7000 photoelectric transducer pixels are available in the market. For increasing the gradation of output images, it is necessary for the linear image sensor to have a wide dynamic range. Some linear image sensors having more than 7000 photoelectric transducer pixels have a dynamic range of three figures or greater and are relatively inexpensive.
When images are read or detected by such linear image sensors, various problems and demands need to be solved and satisfied as described below.
(1) When a high-density subject with a white blank area is read, an offset shift component is generated as an undesirable offset level variation by a pixel transfer array.
(2) The pixels of a linear image sensor may have different sensitivities.
(3) Linear image sensors may contain defective pixels.
(4) The resolution of linear image sensors needs to be increased.
(5) The photodetector of a linear image sensor (joined areas of photoelectric transducer pixels) suffers a curvature in the auxiliary scanning direction.
(6) The odd-numbered pixels and the even-numbered pixels may have different linearity characteristics.
(7) Linear image sensors may be subject to aberrations in the main scanning direction which are caused by the focusing optical system.
(8) Noise may be introduced when a luminance signal is converted into a density signal.
(9) Images represented by output image signals from linear image sensors may contain a coarse feature in a high-density-level area where a luminance signal level is low.